Combines are large agricultural machines that harvest, thresh, separate and clean an agricultural crop. A harvesting assembly harvests an agricultural crop from a field and directs the harvested crop to a feeder house that conveys the harvested crop to a threshing assembly. The threshing assembly threshes the harvested crop removing the grain from the other crop material. The threshed crop material is directed to a separating assembly that expands the threshed crop mat to release trapped grain. The remaining large components of crop material other than grain are expelled from the combine. These large components may be reduced in size by a straw chopper.
Grain and chaff fall from the threshing and separating assemblies and are directed to a cleaning assembly. The cleaning assembly removes the chaff from the grain and blows it out the rear of the combine. The cleaned grain is collected from the floor of the cleaning assembly and is directed to a grain tank by an elevator.
The cleaning assembly comprises a cleaning fan and a cleaning shoe. The cleaning shoe comprises a movable frame holding a series of sieves. The topmost sieve is called a chaffer sieve. The main difference between the chaffer sieve and the secondary sieve located below the chaffer sieve is the size of the sieve openings. The chaffer sieve openings are larger than the secondary sieve. The grain to be cleaned is first deposited on the chaffer sieve. The grain falls through the chaffer sieve to the secondary sieve and from the secondary sieve to the floor of the cleaning assembly. An air blast from the cleaning fan is directed upwardly through the chaffer sieve and the secondary sieve to blow the lighter chaff out the rear of the combine. The frame moves in a reciprocating, shaking or cascading manner to drive the grain to be cleaned longitudinally rearward along the chaffer sieve and secondary sieve. It should be noted that cleaning assemblies may comprise more than one secondary sieve.
Cleaning shoes operate most efficiently when the combine is on level ground. When the combine is operated on side slopes, the grain to be cleaned accumulates on the down hill side of the cleaning shoe and the uphill side is not loaded. In extreme operations, the combine is provided with leveling mechanisms that level the combine during sidehill operations. This is not an economic solution in the vast majority of farming operations. One simple solution has been to provide longitudinally extending dividers on the sieves that limit the downhill migration of grain to be cleaned. This solution is not entirely effective and a variety of other mechanisms a have been proposed that either pneumatically or mechanically drive a portion of the grain to be cleaned to the uphill side of the cleaning shoe.